A protester wearing rudimentary protection against pepper spray is pictured during a confrontation with the police, after a rally for the October 1 'Occupy Central' civil disobedience movement in Hong Kong September 27, 2014. Photo credit: Reuters

Hong Kong police fired repeated volleys of tear gas to disperse pro-democracy protests on Sunday and baton-charged the crowd blocking a key road in the government district after official warnings against illegal demonstrations.

The city's Admiralty district had descended into chaos as chanting protesters converged on police barricades surrounding colleagues who had earlier launched a "new era" of civil disobedience to pressure Beijing into granting full democracy to Hong Kong.

Police, in lines five deep in places and wearing helmets and gas masks, staged repeated pepper spray and shot tear gas into the air. The crowds fled several hundred yards, scattering their umbrellas and hurling abuse at police "cowards".

Crowds returned however and by early evening tens of thousands of protesters were thronging streets, including outside the prominent Pacific Place shopping mall that leads towards the Central financial district.

Fresh rounds of tear gas cleared some of the roads in Admiralty and pushed the crowds towards Central.

Police had not used tear gas in Hong Kong since breaking up World Trade Organization protests against South Korean farmers in 2005.

Clouds of tear gas also blew back towards police lines, but it is unclear how many people on either side have needed treatment.

"We will fight until the end...we will never give up," said Peter Poon, a protester in his 20s, adding that they may have to make a temporary retreat as night falls.

Hong Kong leader Leung Chun-ying pledged "resolute" action against the protest movement known as Occupy Central with Love and Peace.

"The police are determined to handle the situation appropriately in accordance with the law," Leung said just hours before the charge began.

A spokesperson for China's Hong Kong and Macau Affairs Office added that the central government fully supported Hong Kong's handling of the situation "in accordance with the law".

Inside the cordon, thousands had huddled in plastic capes, masks and goggles as they waited for a fresh police charge to clear the area before Hong Kong opens for business on Monday morning.

Hong Kong returned to Chinese rule in 1997 under a formula known as "one country, two systems" that guaranteed a high degree of autonomy and freedoms not enjoyed in mainland China. Universal suffrage was set as an eventual goal.

But Beijing last month rejected demands for people to freely choose the city's next leader, prompting threats from activists to shut down Central. China wants to limit elections to a handful of candidates loyal to Beijing.

'We will win with love and peace'

While promising a fresh round of public consultation, Leung also described Beijing's decision as "legally binding".

Publishing tycoon Jimmy Lai, a key backer of the democratic movement, said he wanted as big a crowd of protesters as possible, after a week of student demonstrations, to thwart any crackdown.

"The more Hong Kong citizens come, the more unlikely the police can clear up the place," said Lai, also wearing a plastic cape and workmen's protective glasses.

"Even if we get beaten up, we cannot fight back. We will win this war with love and peace."

Communist Party leaders in Beijing are concerned about calls for democracy spreading to cities on the mainland, threatening their grip on power. Such dissent would never be allowed on the mainland, where student protests calling for democracy were crushed with heavy loss of life on and around Beijing's Tiananmen Square on June 4, 1989.

Pro-democracy Hong Kong lawmaker Lee Cheuk-yan said three fellow legislators were among a small group of activists detained by police, including democratic leaders Albert Ho and Emily Lau.

Organizers said as many as 80,000 people thronged the streets in Admiralty, galvanized by the arrests of student activists on Friday. No independent estimate of the crowd numbers was available but the action is being seen as the most tenacious civil disobedience action since 1997.

A week of protests escalated into violence when student-led demonstrators broke through a cordon late on Friday and scaled a fence to invade the city's main government compound after a week of peaceful action. Police used pepper spray to disperse the crowd. The Hong Kong Federation of Students extended class boycotts indefinitely.

Police have so far arrested 78 people, including Joshua Wong, the 17-year-old leader of student group Scholarism, who was dragged away after he called on the protesters to charge the government premises. He was still in detention on Sunday.

His parents said in a statement the decision to detain him was an act of "political persecution".

Along with Hong Kong and Chinese officials, some of Hong Kong's most powerful tycoons have spoken out against the Occupy movement, warning it could threaten the city's business and economic stability.